The present invention relates to a portable generator which generates an AC voltage of 100 V or the like by being turned by an engine.
Today, small generators driven by a gasoline engine or a diesel engine, permitting conveyance to where they are needed and capable of developing a per-unit output of several kilowatts, have come into extensive use.
Such portable generators permitting ready conveyance include generators enabled, by keeping the frequency of engine revolutions constant, to develop a single-phase AC voltage of around 100 V in average output voltage at a frequency of 50 or 60 Hz.
However, more recently, there have been proposed systems whereby the output of an engine-driven AC generator is once rectified into a DC voltage and this DC voltage is further converted with an inverter into an output voltage having a constant frequency of 50 Hz or 60 Hz (e.g. JP 63-114527 A and JP 63-302724 A).
Incidentally, an engine-driven small portable generator capable of developing an output of several kilowatts to about 10 kW is not only carried to where they are needed and used for power generation always in a movable state but also may be semi-permanently installed in a fixed position where it is required to operate continually for some time.
This inverter-equipped portable generator, as shown in FIG. 10, has an engine-driven AC generator 50, a DC-voltage-generating circuit 110 using rectifier diodes 115 and thyristors 111, a DC-power-source unit 120 using a large-capacitance capacitor 121 consisting of a required number of capacitors connected in parallel, an inverter circuit 130 using a power transistor, and a low pass filter 140 using a coil and a capacitor.
Further, it has, as control circuits for driving and controlling such power circuits as these DC-voltage-generating circuit 110 and inverter circuit 130, a PWM-signal-generating circuit 250, a voltage-limiting circuit 240, an overload-detecting circuit 260 and an inverter-drive circuit 255. This portable generator 100 also has, as power-supply units for driving these control circuits, a smoothing circuit 210 and a constant-voltage circuit 235.
Many of the AC generator 50 in use whose rotor is turned by such an engine has a three-phase output coil 51 and a single-phase output coil 55. In many cases, the three-phase output coil 51 can develop a maximum output of tens of amperes at hundreds of V, while the single-phase output coil 55 can develop an output of tens of amperes at tens of V.
The DC-voltage-generating circuit 110 to which the output terminal of this three-phase output coil 51 is connected is configured of a rectifier bridge circuit using three rectifier diodes 115 and three thyristors 111. The both output terminals of this rectifier bridge circuit is connect to both ends of the main smoothing capacitor 121, which uses the DC-power-source unit 120, to charge the capacitor 121.
Incidentally, the gate terminal of each thyristor 111 in the DC-voltage-generating circuit 110 is connected to the voltage-limiting circuit 240 to control the continuity angle of each thyristor 111, and the voltages at both ends of the main smoothing capacitor 121, which uses the DC-power-source unit 120, are thereby regulated.
Then, the inverter circuit 130 is configured of a bridge circuit using four power transistors. In this inverter circuit 130, a first transistor 131 and a third transistor 133, arranged in series, are connected to the DC-power-source unit 120, and a second transistor 132 and a fourth transistor 134, arranged in series, are connected to the DC-power-source unit 120. The midpoint between the first transistor 131 and the third transistor 133 is connected to a first output terminal 151 via the low pass filter 140, and the midpoint between the second transistor 132 and the fourth transistor 134 is connected to a second output terminal 152 via the low pass filter 140. Further the base of the first transistor 131 and the base of the fourth transistor 134 are commonly connected to the inverter-drive circuit 255, and the base of the second transistor 132 and the base of the third transistor 133 are commonly connected to an inverter-drive circuit 255.
A first PWM signal supplied from this inverter-drive circuit 255 to the first transistor 131 and the fourth transistor 134 and a second PWM signal supplied to the second transistor 132 and the third transistor 133 are high-frequency pulse signals of several kHz or more. The pulse width of each pulse signal is successively varied between 50 Hz and 60 Hz, and the varying quantity of the pulse width is successively increased or decreased in a sine-wave shape.
Further, the first PWM signal and the second PWM signal are reverse in phase to each other. For this reason, continuity is established between the first transistor 131 and the fourth transistor 134 by the first PWM signal, while discontinuity is ensured between the second transistor 132 and the third transistor 133 by the second PWM signal, and when the midpoint between the first transistor 131 and the third transistor 133 has a voltage VD, which is the voltage of the DC-power-source unit 120, the midpoint between the second transistor 132 and the fourth transistor 134 is at 0 V. When continuity is established between the second transistor 132 and the third transistor 133 by the second PWM signal, the first PWM signal ensures discontinuity between the first transistor 131 and the fourth transistor 134, sets the midpoint between the first transistor 131 and the third transistor 133 to 0 V, and the midpoint between the second transistor 132 and the fourth transistor 134 then to the voltage VD of the DC-power-source unit 120.
This midpoint potential between the first transistor 131 and the third transistor 133 changes over at high speed between 0 V and the voltage VD of the DC-power-source unit 120 as shown in FIG. 11A, and the duration of the DC source voltage VD successively varies. Also, the midpoint potential between the second transistor 132 and the fourth transistor 134 also changes over at high speed between 0 V and the voltage VD of the DC-power-source unit 120 as shown in FIG. 11B, and the duration of the DC source voltage VD successively varies.
As a result, a first output voltage and a second output voltage having passed the low pass filter 140 are are turned into sine-wave voltages of 50 Hz or 60 Hz as shown in FIG. 11. Then, the voltage of the first output terminal 151 and the voltage of the second output terminal 152 are generated as AC output voltages of 50 Hz or 60 Hz, with their peak level and bottom level staggered by a half period.
On the other hand, the single-phase output coil 55 of the AC generator 50 is connected to the smoothing circuit 210 in the control-power-supply circuit as shown in FIG. 10.
This smoothing circuit 210 is configured of a rectifier diode 211 and a smoothing capacitor 215. The rectifier diode 211 is inserted between the output terminal of the single-phase output coil 55 and the smoothing capacitor 215, and the smoothing capacitor 215 is charged with the output voltage of the single-phase output coil 55 to form a DC voltage.
Incidentally, the number of the rectifier diode 211 is not limited to one as shown in FIG. 10, but sometimes four rectifier diodes are used as an all-wave rectifier bridge to charge a smoothing capacitor.
Then, the output terminal of the smoothing circuit 210 is connected to the constant-voltage circuit 235, and this constant-voltage circuit 235 generates a prescribed voltage for driving control circuits.
Moreover, the terminal on the xe2x88x92 side of this constant-voltage circuit 235 is connected to the + side of the DC-power-source unit 120, and the terminal on the + side of the constant-voltage circuit 235 is connected to the voltage-limiting circuit 240, the PWM-signal-generating circuit 250 and an inverter-drive circuit 255.
This voltage-limiting circuit 240 is configured of resistors and comparators. The first reference-voltage resistor 245 and the second reference-voltage resistor 246, arranged in series, are inserted between the + side terminal of the constant-voltage circuit 235 and the + side terminal of the DC-power-source unit 120, and the midpoint between the first reference-voltage resistor 245 and the second reference-voltage resistor 246 is connected to the reference input terminal of a comparator 243. The first voltage-dividing resistor 248 and the second voltage-dividing resistor 249, arranged in series, are inserted between the + side terminal of the constant-voltage circuit 235 and the xe2x88x92 side terminal of the DC-power-source unit 120, and the midpoint between the first voltage-dividing resistor 248 and the second voltage-dividing resistor 249 is connected to the comparing input terminal of the comparator 243.
Further, the output terminal of the comparator 243 is connected to the + side terminal of the constant-voltage circuit 235 via a control resistor 241 as well as to the gate terminal of each thyristor 111 in the DC-voltage-generating circuit 110. In connecting the output terminal of the comparator 243 to the gate terminal of each thyristor 111, it is connected via a protective resistor 117.
Therefore, this voltage-limiting circuit 240 forms a fixed reference voltage by causing the first reference-voltage resistor 245 and the second reference-voltage resistor 246 of the voltage-limiting circuit 240 to divide a fixed voltage generated by the constant-voltage circuit 235 of the control power supply circuit. Further, this reference voltage fixed all the time is entered into the reference input terminal of the comparator 243 of the voltage-limiting circuit 240.
Moreover, in the voltage-limiting circuit 240, a voltage resulting from the addition of the output voltage of the DC-power-source unit 120 and a fixed voltage generated by the constant-voltage circuit 235 is divided by the first voltage-dividing resistor 248 and the second voltage-dividing resistor 249 to form a detection voltage, and this detection voltage is entered into the comparing input terminal of the comparator 243.
As a result, the detection voltage entered into the comparing input terminal varies with the voltage variations of the DC-power-source unit 120 and, if this detection voltage is lower than the reference voltage generated by the first reference-voltage resistor 245 and the second reference-voltage resistor 246, the output of the comparator 243 will be a + potential.
Therefore, the gate potentials of the thyristors 111 can be kept higher than the cathode potentials of the thyristors 111, and a gate current can be supplied to each thyristor 111 via the control resistor 241 to establish continuity of each thyristor 111. For this reason, when the output voltage of the three-phase output coil 51 becomes higher than the voltage of the DC-power-source unit 120, power is supplied to the DC-power-source unit 120 to raise the voltage of the DC-power-source unit 120.
Moreover, when the voltage of the DC-power-source unit 120 rises and the detection voltage entered into the comparator 243 becomes equal to the reference voltage, the output of the comparator 243 becomes 0, and the gate potential of each thyristor 111 becomes equal to the cathode potential. Therefore, each thyristor 111 is placed in a state of discontinuity.
Thus, when the voltage generated by the DC-power-source unit 120 is made lower than a fixed voltage by the voltage-limiting circuit 240, the AC generator 50 performs charging and, when the charged voltage reaches the fixed voltage, stops charging. As a result, it is possible to keep the output voltage of the DC-power-source unit 120 somewhere between 170 V and 200 V to keep the fixed voltage VD set by the voltage-limiting circuit 240 all the time.
Then, the inverter circuit 130 varies the potentials of the first output terminal 151 and the second output terminal 152 in a fixed period of 50 Hz or 60 Hz, and a single-phase AC voltage is supplied with the maximum potential difference between the voltage of the first output terminal 151 and the voltage of the second output terminal 152 being 141 V and the average voltage being 100 V.
The PWM-signal-generating circuit 250 which generates a PWM control signal for controlling this inverter circuit 130 generates the PWN control signal from a reference sine-wave such as 50 Hz, 60 Hz or the like and a triangular wave and supplies it to the inverter-drive circuit 255.
Then, the reference sine-wave of the PWM-signal-generating circuit 250 is generated in accordance with a prescribed frequency, such as 50 Hz or 60 Hz, which is the frequency of the voltage supplied from the output terminal. The PWM-signal-generating circuit 250 regulates the ratio between the voltage of the reference sine-wave and the voltage of the triangular wave, determines the frequency of the pulse signal, which is used as the PWM control signal according to the output voltage VD of the DC-power-source unit 120 entered into the inverter circuit 130 and the characteristics of the inverter circuit 130 and the low pass filter 140, and also determines the pulse width and the pulse width variation.
Moreover, this portable generator 100 is provided with the overload-detecting circuit 260, wherein a detecting resistor 261 is inserted between the DC-power-source unit 120 and the inverter circuit 130.
This overload-detecting circuit 260 is configured of the detecting resistor 261 and an arithmetic-circuit unit 265. When having detected an amperage surpassing the rated amperage, this overload-detecting circuit 260 supplies a stop signal to the inverter-drive circuit 255 according to the extent of surpassing the rating with the time factor also taken into account.
This arithmetic-circuit unit 265 uses various circuits having comparators, capacitors and resistors.
Moreover, taking into account the characteristics of the elements constituting the power circuit, in many cases, the overload-detecting circuit 260 immediately issues a stop signal when a current of double the rated amperage flows to stop the output of the inverter-drive circuit 255 from supplying the first PWM signal and the second PWM signal. On the other hand, the overload-detecting circuit 260 is designed to issue a stop signal to the inverter-drive circuit 255 when it has detected a current slightly surpassing the rated amperage and this current flow has continued for several seconds to several minutes.
Thus, in the portable generator 100, in which a three-phase AC voltage once rectified by the DC-voltage-generating circuit 110 and the DC voltage generated by the DC-power-source unit 120 is a gain converted into an AC voltage by the inverter circuit 130 can generate an AC output voltage whose frequency and voltage are stable all the time while forming a power matching the load by varying the revolutions of the AC generator 50, i.e. revolutions of the engine.
Therefore, this portable generator 100 can adjust the engine revolutions to load variations, increase the revolutions when the load is heavy, and decrease the revolutions when the load is light, thereby making it sufficient for the engine to generate the quantity of energy that the load requires, accordingly can readily adjust the output to the load level, and therefore operate efficiently.
When the portable generator 100 becomes overloaded beyond the rated output, the generator can either stop the inverter circuit 130 from operating promptly or stop the inverter circuit 130 from operating after the lapse of a prescribed length of time, bring down the output voltage to 0 to maintain the overall safety of the circuitry. Moreover, it can operate various electric devices with which the generator is loaded within an extent of several kilowatts as its rated output.
Thus, the engine-driven portable generator 100 using the inverter circuit 130, for its capability to supply single-phase AC power of 100 V as does a commercial power source, has come to be used for supplying power to various electrical devices in general.
Some of such portable generators 100 can perform parallel operation through the adjustment of the output voltage of single-phase AC power and voltage the adjustment of phase.
In such a portable generator 100 capable of performing adjustment of the output voltage and voltage phase, the AC output voltage and AC output amperage supplied from the first output terminal 151 and the second output terminal 152 of the portable generator 100 are detected, and the PWM-signal-generating circuit 250 is controlled, for instance, so as to keep the voltage and phase of the single-phase AC power supplied by this portable generator 100 identical with those of any other generator operating in parallel with it (e.g. JP 5-49174 A, JP 5-236658 A and JP 5-244726 A).
Moreover, the voltage may be adjusted not only in parallel operation but also in independent operation to prevent the voltage from being fluctuating due to the type or magnitude of the load connected to the output terminals (e.g. JP 5-211777 A).
In such a portable generator 100, in many cases, as shown in FIG. 12, the voltage and the amperage of the single-phase AC output supplied from the first output terminal 151 and the second output terminal 152 is detected by inserting an output-voltage-detecting circuit 340 between the first output terminal 151 and the second output terminal 152 following the low pass filter 140 and inserting an output-current-detecting circuit 330 also following the low pass filter 140, and thereby the PWM-signal-generating circuit 250 is controlled.
Incidentally, also in this kind of portable generator 100, as in the portable generator 100 shown in FIG. 10, the single-phase output coil 55 of the AC generator 50 is connected to a control-power-source unit 201 configured of a smoothing circuit 210 and a constant-voltage circuit 235. Therefore, it is also possible to smooth the output voltage of the auk single-phase output coil 55 with the smoothing circuit 210 and to generate a control voltage Vcc of a prescribed level with the constant-voltage circuit 235. However, the +Vcc voltage and the xe2x88x92Vcc voltage, as control voltages, may be generated with the control-power-source unit 201.
Moreover, the output terminals of the three-phase output coil 51 are connected to the DC-voltage-generating circuit 110 as a rectifier bridge circuit using thyristors and rectifying diodes. Therefore, as well as the above-described prior art, a DC voltage is generated by rectifying the output voltage of the three-phase output coil 51 and by charging a large-capacity capacitor as a DC-power-source unit 120, and this DC voltage is entered into the inverter circuit 130 to generate a single-phase AC voltage.
Then, the PWM-signal-generating circuit 250 is configured of a sine-wave-generating circuit 270 for generating a reference sine-wave, a triangular-wave-generating circuit 281 and a PWM-control-signal-generating circuit 285 for generating a PWM control signal. This sine-wave-generating circuit 270 generates an accurate reference sine-wave of 50 Hz or 60 Hz; the triangular-wave-generating circuit 281 generates a triangular wave of a high frequency, such as several kHz to ten-odd kHz; and the PWM-control-signal-generating circuit 285 synthesizes the reference sine-wave and the triangular wave to generate a PWM control signal composed of a pulse string in which the pulse width successively varies.
Further, this sine-wave-generating circuit 270 is configured of an oscillating circuit 271 for supplying a high-frequency signal of several MHz to ten-odd MHz, a go frequency-dividing circuit 273 for generating a clock signal of about 10 kHz by dividing the high-frequency signal supplied by the oscillating circuit 271, a pseudo-sine-wave-generating circuit 275 for generating and supplying a 50-Hz- or 60-Hz-stepwise sine-wave by generating many different potentials with a multi-stage voltage-dividing resistor and by successively selecting the different potentials with a multiplexer operating in response to a clock signal, a voltage-regulating circuit 277 for regulating the peak voltage of the stepwise sine-wave supplied by the pseudo-sine-wave-generating circuit 275, and a low pass filter 279 for generating a smooth sine-wave from the stepwise sine-wave.
Moreover, a voltage detection signal supplied from the output-voltage-detecting circuit 340 is entered into a square-wave-forming circuit 291 to generate a square wave signal having zero cross points of the AC output voltage as its leading edge and trailing edge. Then, the zero cross signal, which is a square wave signal, is entered into a start-timing circuit 293 and a phase-comparator circuit 297.
Then, the start-timing circuit 293 causes the pseudo-sine-wave-generating circuit 275 to supply the pseudo-sine-wave by undoing the reset stage of the pseudo-sine-wave-generating circuit 275 in the sine-wave-generating circuit 270.
If the output-voltage-detecting circuit 340 detects any voltage variation between the first output terminal 151 and the second output terminal 152 when, this pseudo-sine-wave-generating circuit 275 being in a reset stage, no reference sine-wave is supplied from the sine-wave-generating circuit 270, i.e. when the inverter circuit 130 is not working, the start-timing circuit 293 releases the pseudo-sine-wave-generating circuit 275 from its reset state in response to a zero cross signal from the square-wave-forming circuit 291. Therefore, the phase of the reference sine-wave supplied by the sine-wave-generating circuit 270 and the phase of the voltage generating between the first output terminal 151 and the second output terminal 152 can be made identical to each other.
To add, in starting the operation of the pseudo-sine-wave-generating circuit 275, even if the zero cross signal is not entered into the start-timing circuit 293 within a prescribed period of time, the reset state of the pseudo-sine-wave-generating circuit 275 is undone to cause the sine-wave-generating circuit 270 to start supplying a reference sine-wave.
Then, a current detection signal from the output-current-detecting circuit 330 is entered into a square-wave-forming circuit 295, an overload-detecting circuit 269 and a limit-value-detecting circuit 299. The square-wave-forming circuit 295 generates a zero cross signal matching the phase of the output current; the overload-detecting circuit 269 generates a stop signal when the rated amperage has been surpassed; and the limit-value-detecting circuit 299 generates a voltage regulating signal when an amperage not above the rated amperage goes out of the range between prescribed lower and upper limits.
This square-wave-forming circuit 295, on the basis of a current detection signal supplied from the output-current-detecting circuit 330, generates a square wave signal whose leading edge and trailing edge are zero cross points of the AC output amperage. Then, it enters this square wave signal, as a zero cross signal, into the phase-comparator circuit 297.
Moreover, the phase-comparator circuit 297 compares the phase of the output current with the phase of the output voltage according to the zero cross signal based on the current detection signal and the zero cross signal based on the voltage detection signal. If the current phase is behind the voltage phase, it supplies an addition signal as a phase regulating signal to the frequency-dividing circuit 273. On the other hand, if the current phase is ahead of the voltage phase, it supplies a subtraction signal as a phase regulating signal to the frequency-dividing circuit 273.
Then, the frequency-dividing circuit 273 in the sine-wave-generating circuit 270, when generating a clock signal of several kHz to ten-odd kHz by dividing a high-frequency signal, adds one pulse at every few hundreds of pulses of the clock signal if an addition signal is entered from the phase-comparator circuit 297. On the other hand, if a subtraction signal is entered from the phase-comparator circuit 297, it generates a clock signal by subtracting one pulse at every few hundreds of pulses of the clock signal.
In this way, the portable generator 100 in which voltage fluctuations are prevented, when the current phase is behind the voltage phase, slightly advances the phase of the pseudo-sine-wave and, accordingly, the phase of the reference sine-wave by increasing the pulses of the clock signal. On the other hand, when the current phase is ahead of the voltage phase, it slightly delays the phase of the reference sine-wave by subtracting pulses of the clock signal. Thereby the phase of the PWM control signal is adjusted so as to adjust the phase of the single-phase AC voltage supplied by the portable generator 100.
Moreover, the overload-detecting circuit 269 into which the current detection signal supplied from the output-current-detecting circuit 330 is entered, on the basis of the current detection signal supplied from the output-current-detecting circuit 330, immediately issues a stop signal if the rated amperage is greatly surpassed. On the other hand, it performs time integration and issues a stop signal after a prescribed length of time if the rated amperage is only slightly surpassed. Then, this stop signal is entered into a voltage-control circuit 240 and the inverter-drive circuit 255, resulting in cutting off a gate current supplied by the voltage-control circuit 240 to stop the operation of the DC-voltage-generating circuit 110, and also to stop the supply of the first PWM signal and the second PWM signal supplied by the inverter-drive circuit 255. Thereby the operation of the inverter circuit 130 is stopped.
Further, the limit-value-detecting circuit 299 into Apt; which the current detection signal supplied by the output-current-detecting circuit 330 is entered is a circuit in which the upper limit and lower limit of amperage are set. This limit-value-detecting circuit 299 supplies a voltage regulating signal to the voltage-regulating circuit 277 to lower or raise the peak level (amplitude) of the reference sine-wave so as to slightly increase the output voltage between the first output terminal 151 and the second output terminal 152 when the amperage of the current detection signal has dropped below the lower limit of amperage.
Thus, in the portable generator 100 provided equipped with the limit-value-detecting circuit 299, since it is enabled to finely adjust the output voltage by setting the upper limit of amperage and the lower limit of amperage within the range of the rated amperage and by adjusting the duty ratio of the first PWM signal and the second PWM signal, under the condition in which plural generators are operating in parallel, the load can effectively be shared on each portable generator 100, either by slightly raising the output voltage to increase the output current if the shared load is small or by slightly reducing the output voltage if the current fed to the load is near the limit of the rated amperage.
Moreover, in the portable generator 100, the output voltage varies according to the capacity or the type of the load when the portable generator 100 is not operated in parallel with any other generator, i.e. when the portable generator 100 is used by itself in independent operation. Thus, the portable generator 100 is sometimes made to stabilize the level of the single-phase AC voltage, as the output voltage, by regulating either the amplifying rate of the voltage-regulating circuit 277 or the voltage of the triangular wave supplied from the triangular-wave-generating circuit 281 on the basis of the peak voltage detected by the output-voltage-detecting circuit 340.
As described above, the portable generator, generating an AC voltage with an engine-driven three-phase generator, rectifying the AC voltage with a DC-voltage-generating circuit using thyristors, charging a DC-power-source unit using a large-capacitance capacitor, and generating a single-phase AC voltage of a fixed frequency and of a fixed voltage from the DC voltage supplied from this DC-voltage-generating circuit with inverter circuit, can easily and effectively provide a single-phase AC voltage of a fixed voltage.
However, an engine-driven portable generator 100 may suffer a drop in output voltage when the load is heavy and the output power nears the rated output capacity of the portable generator.
Moreover, also proposed is a portable generator 100 which, when a heavy load is connected thereto, prevents the output voltage from dropping by adjusting the pulse width of the PWM signal. However, this also involves the problem that, if the output voltage level is maintained by altering the pulse width of the PWM singal when the output current is increased, heat emission from the power circuit will increase, resulting in a fall in the conversion efficiency of the engine output to the output electric power.
This problem that arises when the output current increases to invite a heavily loaded state approaching the rated output capacity of the portable generator means that, as shown in FIG. 13, when the load increases with the output voltage V being kept constant, the output current A increases and the output voltage VD of the DC-power-source unit varies. For this reason, when this DC voltage VD is converted into a single-phase AC voltage in accordance with a PWM signal of the same fixed pulse width as that under a light load, there arises a drop in output voltage. Then, when the DC voltage, which is the input voltage into the inverter circuit, slightly drops, if it is attempted to maintain the output voltage from the output terminals by altering the pulse width of the PWM signal, increasing the current in flow into the inverter circuit and raising the duty ratio of the output voltage of the inverter circuit, the heat emission from the power circuit rapidly increases.
Then, this pulsation of the DC voltage can be reduced by increasing the capacitor capacitance of the DC power source unit. The present applicant has achieved the object of providing a portable generator capable of preventing the output voltage from varying even under a heavy load and efficiently respond to such a load by enhancing the efficiency of the rectifier circuit, which is the DC-voltage-generating circuit using thyristors.
The present invention provides a portable generator (100) generating an AC voltage with an AC generator (50) turned by an engine, rectifying this AC voltage by a thyristor-based rectifier circuit which serves as a DC-voltage-generating circuit (110) to charge a DC-power-source unit (120), converting the DC voltage, which is generated by charging the DC-power-source unit (120), into a single-phase AC voltage of a prescribed frequency and of a constant voltage by an inverter circuit (130) to supply from output terminals (151 and 152); which is provided with a constant-voltage-control unit (500) that controls to keep the DC voltage of the DC-power-source unit (120) substantially constant by detecting the voltage at the DC-power-source unit (120) and subsequently by controlling the continuity angle of thyristors (111) in the DC-voltage-generating circuit (110), and that controls, by detecting the amperage flowing in the inverter circuit (130), to advance the start of continuity establishment of the thyristors (111) when the amperage flowing in the inverter circuit (130) increases.
Thus, since the constant-voltage-control unit (500) is provided to advance the start of continuity establishment of the thyristors (111) if the amperage of the current flowing in the inverter circuit (130) increases, the current to charge the DC-power-source unit (120) is increased by the DC-voltage-generating circuit (110) if the load is heavy and the output current is great, so that, even if the load current is large, the DC voltage supplied by the DC-power-source unit (120) can be prevented from dropping and that the voltage supplied from the output terminals (151 and 152) can be prevented from dropping.